The present invention relates to electroencephalography and more particularly to the suppression of the confounding effects of artifacts resulting from eye and head movements (class 1282.1 B).
At the present time it is known that it is possible to observe and record a subject's brain waves by attaching a number of removable electrodes to the subject's scalp. The subject's brain waves, at the present time, are generally recorded on a paper strip chart in analog form and analyzed by being viewed by a trained clinical neurologist.
It has also been suggested that subjects' brain waves may be analyzed by various analog type computers and by utilization of digital computers using special algorithms.
If the subject under tests moves or blinks, or even if his eyes move, the electrodes connected to his scalp may detect a large electrical response which may be confused as being a brain wave signal. A movement artifact may be distinguished from a brain wave by one or another of the following characteristics: (i) it is a large signal beyond normal excursion limits, (ii) it is unusually widespread and rapid event, as shown by its first derivative, or (iii) it has a power spectrum with most of the energy between 0.5 and 1.1 Hz.
In some systems of testing it is desirable to obtain the subject's "evoked response," the subject's brain waves, in response to a known and selectively timed stimulus such as a light flash or an auditory click. It may happen, however, that the stimulation which gives rise to the brain wave response also also provokes a movement artifact, that is, the subject may blink, move his eyes or his head, creating an electrical signal which may be mistaken as a brain wave. Such artifacts usually appear at the same time across multiple recording electrodes.
The literature discloses that artifact compensation in EEG (electroencephalography) has been accomplished through a number of methods. In 1975 Huang and Flynn suggested placing the detection electrode a distance of 3-4 mm from the signal source to achieve pronounced muscle artifact attenuation, see Electroencephalography and Clinical Neurophysiology, "Recording of Single Unit Activity During Electrical Stimulation and Microintophoresis, 1977." Rex Y. Wang and George K. Aghajahian suggest use of a differential amplifier in combination with a recording and an indifferent electrode so that the common signal, noise-artfact, is canceled (Id). They also suggest placing two detection electrodes in close opposition to the signal source to achieve the ideal differential recording in which the stimulus artifacts will match and cancel (Id). Artifact distortion may be reduced by signal averaging or common mode identification methods, but it has been suggested that neither is adequate where the signal is continuous. Journal of Electrophysiological Technology, "Computers in Neurophysiology," C. D. Binnie.
In U.S. Pat. No. 3,893,450 entitled "Method and Apparatus for Brain Waveform Examination," there is mentioned, at column 1, lines 36-41, that artifacts that impede the measurement of brain responses to stimulation are one of the four general problems in accurate EEG analysis. In U.S. Pat. No. 3,774,593 entitled "Method of and Apparatus For Sleep Monitoring By Brainwave Electromyographic and Eye Movement Signals" there is disclosed a set of two electrodes for picking up eye movement (EM) secured on the temple portions of the subject. A special electrical cable design to suppress artifact is described in Pryzbylik, "Low Noise Cable and Slip Ring Assembly For Recording of Small-Amplitude Physiological Signals in Chronic Animals," Medical and Biological Eng., Vol. 14, No. 5, pp. 565-569, Sept. 1976. The article Gevins et al, "Automated Analysis of the Electrical Activity Of the Human Brain (EEG): A Progress Report, "IEEE Proceedings, Vol. 63, No. 10, Oct. 1975, at page 1392 generally describes a method of artifact rejection in which the intensity of the electrical signal representing the artifact is compared with a threshold value set during initial calibration. The data being collected is discarded until the voltage representing the artifact falls to below the threshold value.
The use of various systems of electrode placement, including the use of an electrode cap as shown in U.S. Pat. No. 3,998,213, has been suggested and the problem of electrode systems for active subjects such as hyperactive children has also been explored, see Hanley et al, "Electrode Systems For Recording The EEG In Active Subjects," Biomedical Electrotechnology, Academic Press 1974. However, the use of special electrodes and the placement of the electrodes does not markedly reduce the adverse effects from the subject's movement. In U.S. Pat. No. 3,841,309 an "adjustable threshold device" is used in an EEG system to determine the timing between brain wave detected peaks in an EEG channel.